The related application mentioned above discloses a television tuning system in which coarse tuning information, fine tuning information and equalization information for selected television channels are stored in appropriate memories. The equalization information is determined from the tuning voltage-versus-frequency characteristic of the tuner and represents the slope of the tuning curve in the vicinity of the frequency corresponding to the selected channel number. The memories are interrogated for each channel change and the respective informations read out and combined to produce a signal from which a tuning voltage may be generated. The application alludes to equilization of an automatic frequency control potential but does not disclose apparatus therefor.
As shown in the prior art (and as described in the above-entitled application) the memories are interrogated by means of a comparator and counter arrangement. One set of comparator inputs is supplied by the memory and the other is supplied by the counter. The counter is clocked until its binary count matches the binary content of the accessed memory location--as determined by the comparator. The output of the comparator is at a first logic level whenever the count of the counter is less than the binary count stored in the accessed memory location and at a second logic level whenever the count of the counter is equal to or greater than the binary content of the memory. In the circuit of the above mentioned copending application the counters are clocked for operation in fixed time intervals and the output of the comparators remain at a 0 logic level for the time it takes to reach the stored binary content. Thus a 0 level pulse is generated, the width of which is directly related to the time required for the counters to attain the stored count, which is a direct indication of the information stored in the memory. The comparator output is thus a PWM signal indicative of the memory content.
While circuits are known in the art for combining pulse-width modulated tuning signals to produce a common tuning voltage output, none combines coarse tuning information with equalized fine tuning information and equalized AFC information for producing a tuning voltage.